[GL-MT300N-V2] slow wifi speed

Hi, after installing the latest firmware (3.102) I noticed that when coping files (from my pc to a ssh shared folder on my home nas) the transfer speed through wifi decreased from 9MB/s to 5.5MB/s. Using a cable the speed is as usual 9MB/s.
What could be the problem?

Actually the speed is not bad. I am not sure if it is a firmware problem.